The present invention relates to gas analyzing apparatus in general and in particular to an optical path length modulated absorption-absorption trace gas detection apparatus with means for providing compensation for reflection losses.
In U.S. Pat. No. 3,679,899 issued to the applicant and assigned to the assignee of the present application, there is described several techniques and apparatus for detecting trace quantities of gas by means of non-linear mixing in two samples of the gas to create a cross-correlation (or heterodyne) signal. The techniques described in the patent depend on dynamic changes of the total absorption of radiant energy in a reference gas sample and in a sample of gas containing an unknown quantity of the reference gas, with the dynamic changes being produced by changing the density of the gas samples in containers of fixed optical path length or by changing the optical path length in a pair of containers wherein the gas is maintained at a fixed density. The former technique requires relatively large volumes and powers to allow suitable fractional modulation of density. The latter technique suffers from adverse effects on signals due to the effect of variable optical geometry and the resulting effects introduced by variable reflection from the various optical surfaces involved.